Quest for understanding neutron emission in nuclear fission: The case of ${}^{210}\mathrm{Po}$

The neutron emission, extensively used as a clock to measure fission delay, is not fully understood. There are ambiguities in interpretation of experimental results at higher energies and a dearth of information at low energies. In order to address these issues, a measurement of pre-scission neutron multiplicity $\left({\nu }_{\mathrm{pre}}\right)$ for ${}^7\mathrm{Li}+{}^{203}\mathrm{Tl}$ at low energy $(E^{*}=43.7\mathrm{MeV})$ have been carried out. The statistical model calculations incorporating shell corrections, constrained by fitting fission excitation functions down to energies near fission threshold, underestimate the experimental ${\nu }_{\mathrm{pre}}$ values. At the energy of the present measurement, dynamical effects are found to be negligible. This would imply large emission of neutrons at the instant of neck rupture. More theoretical and experimental investigations are required to understand the behavior of low density neutron rich nuclear matter at the neck region.

Figure 1

The schematic of the experimental setup consists of a silicon strip detector (SSD) for detection of fission fragments and an array of 15 liquid scintillator (LS) detectors for neutron detection.

Figure 2

(a) A typical energy spectrum recorded in one of the strips of the silicon strip detector. (b) Pulse shape discrimination (PSD) vs. time of flight (TOF) spectrum from one of the liquid scintillator detectors.

Figure 3

Typical neutron energy spectra along with the moving source fits. The compound nucleus (CN), detected fragment (DF), and complementary fragments (CF) contributions are also shown. The numbers in brackets represent the angle of neutron emission with respect to the beam, DF, and CF in laboratory frame, respectively.

Figure 4

The energy integrated neutron yields plotted as function of relative angle (${\theta }_{nDF}$) between the emitted neutron and detected fragment (DF) for different angular bins of DF (${\theta }_{DF}$). The results of moving source fit for compound nucleus (CN), detected fragment (DF), complementary fragment (CF), and the total correspond to mean fragment angle (${\theta }_{DF}={150}^{\circ }$). The shaded region represents the variations in the contributions considering the range of fragment detection angles.

Figure 5

The experimental fission probabilities for ${}^{210}\mathrm{Po}$ in $\alpha$ [32], ${}^{12}\mathrm{C}$ [29, 30], and ${}^{18}\mathrm{O}$ [29, 31] induced reactions are compared with the results of different types of statistical model calculations (see text for details). The continuous, dot-dashed and dashed lines correspond to Types I:A, II:A, and II:B calculations, respectively. The dotted line corresponds to Type-I:B calculation for ${}^{12}\mathrm{C}+{}^{198}\mathrm{Pt}$ reaction. The probabilities are scaled for representation purpose.

Figure 6

The comparison of experimentally measured ${\nu }_{\mathrm{pre}}$ values for ${}^{210}\mathrm{Po}$ in ${}^7\mathrm{Li}$ (present data), ${}^{12}\mathrm{C}$ [18, 38], and ${}^{18}\mathrm{O}$ [39] induced reactions. The statistical model predictions in Types I:A, I:B, II:A, and II:B calculations for ${}^{12}\mathrm{C}$ and Type-I:A calculation for ${}^{18}\mathrm{O}$ induced reactions are shown (see text for details). The predictions of the vecstat code with $\beta =0$, $3{\mathrm{zs}}^{-1}$ from Ref. [40] are also plotted for comparison. The pre-saddle ${\nu }_{\mathrm{pre}}$ extracted in Ref. [12] are also shown.